The practice of electrostatic spraying of protective liquid coating materials such as paint is well established and commercially practiced. In recent years, however, there has been a great deal of interest and development in electrostatic spraying of solid particulate materials. These so-called powder spraying systems enable solid paints or other coating materials to be applied to a substrate without a liquid carrier or solvent. Advantages of powder spraying systems over liquid spraying systems include eliminating the cost of the liquid carrier, increased ease of application and control, less expensive application equipment, a wider range of film thickness capability and the elimination or at least a dramatic reduction of exhaust gases expelled to the atmosphere.
Electrostatic powder spray systems operate on the principle of transporting a finely divided powder, generally on the order of from 10 to 50 microns, to a spray gun or spray head while entrained in an air or gaseous stream. The powder is subsequently transferred from the gun to the substrate by an electrostatic charge applied to the powder and an opposite charge on the substrate. Once applied to the substrate the powder is generally adhered as a film by heat fusion.
Powder spraying systems such as disclosed in U.S. Pat. No. 3,960,323 and other patents assigned to the assignee of this invention, are currently used for many applications and are gaining widespread acceptance commercially. Such systems generally include a powder spray booth having top, bottom, side and end sections in which a plurality of electrostatic spray guns and a powder filtering system are disposed. In most known systems, the top of the booth is formed with a slot to receive an endless conveyor along which a plurality of articles to be coated are disposed. Each of the articles is suspended from the conveyor by one of a plurality of spaced hooks or other convenient means to place them in general alignment with the electrostatic spray guns. The spray guns are disposed at selected locations within the powder booth to create a uniform spray pattern of powder so as to assure each of the articles passing through the spray booth receives a complete and even coating of powder.
While the process of electrostatic spraying of solid particulate materials has been applied to a variety of articles, prior to this invention the coating of essentially continuous or elongated articles such as pipe, rods or wire has generally been conducted by other methods such as the electrostatic spraying of a liquid coating. For the reasons outlined above, coating with solid particulate materials is much more desirable than using liquid materials. However, existing designs of powder spray systems are incapable of efficiently accommodating continuous elongated articles. While commonly used conveyor systems such as described above for moving articles through a powder spray booth could conceivably be adapted to transport pipe, rods, wire or similar articles, it is probably that only a single article of this nature could be handled at one time to assure the proper coating is applied. Particularly for small guage wire or similar articles, this would be an extremely inefficient and uneconomical use of existing powder spray booths.
It has therefore been an object of this invention to provide a method and apparatus for the powder coating of elongated articles.
It is another object of this invention to provide a method and apparatus for the simultaneous coating of a plurality of pipe, rod, wire or similar elongated articles by electrostatic spraying of solid particulate materials.
It is another object herein to provide a method and apparatus for the simultaneous electrostatic spraying of solid particulate materials to form a coating on a plurality of elongated articles in which the spraying pattern of electrostatic spray guns is carefully controlled to assure complete and uniform coating of such articles.